kern/140597: Lost Retransmission Detection

Richard Scheffenegger rs at
Mon Nov 16 07:20:02 UTC 2009

>Number:         140597
>Category:       kern
>Synopsis:       Lost Retransmission Detection
>Confidential:   no
>Severity:       serious
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Mon Nov 16 07:20:00 UTC 2009
>Originator:     Richard Scheffenegger
>Release:        8.0-RC2
FreeBSD rsFreeBSD.vie.demo 8.0-RC2 FreeBSD 8.0-RC2 #16: Sat Nov 14 22:25:28 CET 2009     root at rsFreeBSD.vie.demo:/usr/obj/usr/src/sys/GENERIC  i386

SACK TCP in FreeBSD does not do Lost Retransmission Detection.

Unlike linux, that functionality is not implemented, leading to a RTO when a single segment is lost twice. This reduces Goodput considerably during phases 
of higher congestion.


Drop the same segment of a TCP Stram twice, when cwnd is open enough (3+ segments).
Simple fix is rather easy (doesn't work perfectly under all circumstances,
but doesn't need new state variable and runs in constant time):

When sending the last segment in a sackhole, store snd.max in rxmit.
During SACK Ack processing, check the value of rxmit from the lowest 
hole against snd.fack; if snd.fack <= rxmit of that hole, set rxmit to 
start, and sackhint.nexthole == sackhole[0]. 

Care must be taken during hole-splitting (if rxmit > end of lower hole, set
rxmit to snd.max), and all checks of sackhole[].rxmit need to deal with
the signal rxmit > end.

See ff.


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